RFR: 8366971: C2: Remove unused nop_list from PhaseOutput::init_buffer

[Daniel Jeliński]

On Tue, 9 Sep 2025 08:01:40 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> The nop list has never been used in the history of OpenJDK. Let's clean it up.
>> 
>> Tested with Mach5 tier 1-5, no related failures.
>
> src/hotspot/cpu/ppc/ppc.ad line 4926:
> 
>> 4924:   // Unused, list one so that array generated by adlc is not empty.
>> 4925:   // Aix compiler chokes if _nop_count = 0.
>> 4926:   nops(fxNop);
> 
> There seems to be some justification here why we needed to have the list.
> Can you quickly say why we should not be worried about that now? ;)

I don't have the AIX compiler at hand, but based on the comment I'd guess that the AIX compiler errored out either on [this](https://github.com/openjdk/jdk/blob/b1fa1ecc988fb07f191892a459625c2c8f2de3b5/src/hotspot/share/opto/output.cpp#L1403) or on [this](https://github.com/openjdk/jdk/blob/91f12600d2b188ca98c5c575a34b85f5835399a0/src/hotspot/share/adlc/output_h.cpp#L1122). Both these lines are removed in this PR.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27117#discussion_r2332483194